This invention relates to a coating apparatus of the extrusion type adapted to form a film of coating liquid on a flexible sheet material continuously traveling and a coating method using such a coating apparatus, and more particularly to a coating apparatus and a coating method which are suitable for use for manufacturing of a magnetic tape or the like.
A coating apparatus of the extrusion type has been extensively directed to a variety of applications such as manufacturing of a magnetic tape and the like because it is superior in both operability and controlling of a thickness of a coated film to other coating apparatus utilizing roll coating techniques, gravure coating techniques, doctor coating techniques and the like. However, the coating apparatus of the extrusion type causes stagnation of coating liquid to relatively frequently occur in a flow passage through which the coating liquid is fed. Such stagnation of the coating liquid tends to lead to a variation in viscosity of the coating liquid and coagulation or flocculation thereof, resulting in streaks and/or color shading being generated on a film formed of the coating liquid.
A coating apparatus intended to solve such a problem as described above is disclosed in each of Japanese Patent Application Laid-Open Publication No. 236968/1989, Japanese Patent Application Laid-Open Publication No. 238362/1986 and Japanese Patent Application Laid-Open Publication No. 95174/1987. More specifically, the coating apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 236968/1989 is so constructed that a part of coating liquid received in a pocket of a coater is withdrawn out of the pocket at a position thereof farthest from a coating liquid feed position of the pocket in a width direction of a flexible sheet material to be coated with the coating liquid, resulting in stagnation of the coating liquid in the pocket, as well as both a variation in viscosity of the coating liquid and flocculation thereof being prevented.
However, the coating apparatus of the Japanese publication necessarily requires a withdrawal piping system, a withdrawal pump, a filter unit, a flow meter and the like for forcibly withdrawing the coating liquid out of the pocket of the coater, resulting in being complicated in structure. Also, it requires to control, within a predetermined narrow range, a ratio between a rate at which the coating liquid is fed into the pocket and a rate at which it is withdrawn therefrom for coating. Such control is highly troublesome. Also, a failure in such control or deviation of the ratio from the predetermined range causes air to enter the pocket, resulting in a coated film or a film formed of the coating liquid being defective.
The coating apparatus of Japanese Patent Application Laid-Open Publication No. 238362/1986 is constructed so as to prepare a magnetic paint by mixing a paint containing a magnetic powder and a curing agent with each other, guide the magnetic paint to a swirl flow passage to subject it to a swirling treatment, and then apply it onto a non-magnetic support or carrier. Unfortunately, the coating apparatus disclosed tends to cause streaks or color shading to occur on a coated film formed of the coating liquid, to thereby fail to improve surface properties of the coated film.
The coating apparatus of Japanese Patent Application Laid-Open Publication No. 95174/1987 is so constructed that a rotary unit is received in a pocket of a coater to subject coating liquid to shearing, to thereby keep viscosity of the coating liquid in a width direction thereof substantially constant. The coating apparatus requires to arrange a rotary cylinder in the pocket of the coater, leading to large-sizing of the coater and complication in structure thereof. Also, it needs a drive system for driving the rotary cylinder, to thereby cause an increase in manufacturing cost thereof. Further, it causes deflection of the rotary cylinder, so that application of the coating liquid to a sheet material of an increased width is highly difficult.